Photocurable compositions are used in forming printing plates and other photosensitive articles. In the field of photosensitive flexographic printing plates, the uncured plate typically includes a support and a photocurable surface or layer. Additional layers or surfaces on the plate can include, for example, slip films and/or release layers to protect the photocurable surface. Prior to processing the plate, the release layer typically is removed, and the photocurable layer is exposed to radiation in an imagewise fashion. Unexposed areas of the photocurable layer are then removed in developer baths.
In the past, unexposed areas of the photocurable layer were removed using developer baths comprising organic solvents. However, the toxicity, volatility, and low flash point of such solvents give rise to hazardous conditions and pollution problems. As a result, there has been a recognition of the need to provide photocurable composition which can be developed in, for example, aqueous solvents.
A flexographic printing plate not only should possess a photocurable layer, but also should have sufficient flexibility to wrap around a printing cylinder and sufficient structural integrity to withstand the rigors experienced during typical printing processes. The printing plate should also be soft enough to facilitate ink transfer during printing, and should exhibit solvent resistance to inks, including resistance to the various organic and aqueous-based inks which are used in flexographic printing.
Several approaches have been taken to answer the need for aqueous developable compositions for flexographic printing. One approach, described in European Patent Application No. 552,799 Al, involves the use of a two-step polymerization process to form a core/shell structure wherein a crosslinked shell is polymerized onto an elastomeric core. Formation of a core/shell structure generally requires two polymerization steps, and two monomer mixtures. Another approach, described in European Patent Application No. 513,493 Al, involves the use of a composition comprising a hydrophilic polymer.
There remains a need for aqueous developable polymers which can, if desired, be formed in a single polymerization step and which allow for flexibility in the choice of monomers. Such polymers should maintain an appropriate balance of properties such as flexibility and structural integrity, while being resistant to organic and aqueous-based inks. The present invention is directed to these and other important ends.